Voltage regulator circuits or voltage regulators are widely used in many applications to provide a nearly constant output voltage at a desired level that is substantially independent of a poorly specified and often fluctuating input voltage and output conditions (i.e., variation in a load current).
One type of voltage regulator is a replica transistor voltage regulator. In a replica transistor voltage regulator a voltage established in a replica leg using a dummy or replicated load and is replicated in an output leg to provide a desired output voltage (Vout). Typically, the output leg is made using larger semiconductor devices capable of carrying higher current demanded by devices or circuits coupled to an output-node of the regulator. Vout from the output-node in the output leg is regulated substantially independent of an output load by forcing the output leg to track voltage in the replica leg as closely as possible.
An example of a conventional replica transistor voltage regulator is shown in FIG. 1. Referring to FIG. 1, the voltage regulator 100 includes an operational amplifier (OPAMP 102) having a non-inverting input coupled to a reference voltage (Vref), a replica leg 104 coupled between a voltage source (Vpwr) and a circuit ground 106, and an output leg 108 coupled between Vpwr and an output-node 110. The replica leg 104 includes a replica transistor 112 coupled to and controlled by a voltage (Vgate) output from the OPAMP 102, and a replicated or dummy load 114, represented here as a resistance (Rrep) and a parallel capacitance (Crep), through which the replica transistor 112 is coupled to ground 106. The output leg 108 includes a second, typically larger output transistor 116 coupled to the OPAMP 102 and controlled by Vgate, and the output-node 110 through which the output transistor is coupled to an output load, represented here by a current (Iload) and a capacitance (Cload). The OPAMP 102 is configured in negative feedback so that the output of the OPAMP, Vgate, forces the Vout voltage to the same voltage as a voltage (Vrep) in the replica leg 102. The replica transistor 112 and output transistor 116 are ratioed so that the current provided to output leg 108 is much larger than that of the replica leg 104 at the desired output voltage.
Although the above described circuit is widely used, and has the advantages of a simple architecture that occupies a small area on a silicon die or substrate, it is not wholly satisfactory for a number of reasons. In particular, conventional replica transistor voltage regulators suffer from poor accuracy, typically allowing the output voltage to vary by about 7-10% or more from a desired output voltage, making it unsuitable for use in many circuits.
An alternative voltage regulator architecture further includes a current conveyor coupled between the first leg of the circuit and an output-node in the replica leg. The current conveyor provides feedback between an output voltage (Vout) and an operational amplifier (OPAMP) at the input to the voltage regulator. The OPAMP controls current supplied to the current conveyor based on a comparison between a reference voltage and a feedback voltage. The current conveyor forces Vout to follow the input or source voltage. Although voltage regulators including current conveyors provide regulation with a relatively good accuracy in output voltage, typically varying by as little as 5%, they too suffer from a number of drawbacks or disadvantages including poor headroom of less than about 50 millivolts (mV), and a poor power supply rejection ratio (PSRR), typically of about −5 decibels (dB) or greater. By headroom it is meant a maximum allowable shift in input or source voltage for which the voltage regulator can adjust or compensate in the output voltage Vout. PSRR is a term widely used in the field of electronics to quantify noise coupled from a power supply to a considered node, such as the output-node. More fundamentally, the current conveyor architecture requires a relatively large area on a die or substrate on which it is fabricated, utilizing from about 133K to 150K square microns (μm2), making it unsuitable for use in many integrated circuits (ICs).
Accordingly, there is a need for a voltage regulator that does not suffer from the above shortcomings of conventional designs and methods. In particular, there is a need for a highly accurate voltage regulator that has a good PSRR and no headroom limitations while occupying a small area on the substrate on which it is fabricated.